$-3bcd + 9c + 9d + 5 = c - 10d + 4$ Solve for $b$.
Combine constant terms on the right. $-3bcd + 9c + 9d + {5} = c - 10d + {4}$ $-3bcd + 9c + 9d = c - 10d - {1}$ Combine $d$ terms on the right. $-3bcd + 9c + {9d} = c - {10d} - 1$ $-3bcd + 9c = c - {19d} - 1$ Combine $c$ terms on the right. $-3bcd + {9c} = {c} - 19d - 1$ $-3bcd = -{8c} - 19d - 1$ Isolate $b$ $-{3}b{cd} = -8c - 19d - 1$ $b = \dfrac{ -8c - 19d - 1 }{ -{3cd} }$ Swap the signs so the denominator isn't negative. $b = \dfrac{ {8}c + {19}d + {1} }{ {3cd} }$